Hitherto, in optical recording medium handling devices typified by an optical disk recorder/player, the diameter (φ) of a focal spot is generally given according to formula (1):φ=λ/NA   (1)where λ is the wavelength and NA is the numerical aperture of the objective lens.
Therefore, the shorter the wavelength of the light source and the larger the numerical aperture of the objective lens, the smaller the diameter of the focal spot on the recording medium and the higher the possible density of optical recording.
A focusing objective lens used for an optical data recording/playback device is designed generally so that the residual wavefront aberration is minimized for a cover layer (transparent protective layer) which is provided as a protective film on a data-recording layer of a recording medium and which has a specific thickness. For example, a focusing objective lens is designed so as to be the optimum for a cover layer thickness of 1.2 mm in a CD (compact disk) device, and 0.6 mm in a DVD (digital versatile disk) device.
There is provided an optical disk (so-called multi-layer disk) in which a plurality of data-recording layers are laminated. In this multi-layer disk, each data-recording layer is covered by a cover layer, the cover layers having different thicknesses from each other.
In a DVD playback device that plays back a DVD having two data-recording layers, the numerical aperture of an objective lens is 0.6, and a red semiconductor laser with a wavelength of 650 nm is used as a light source.
The tolerance of the objective lens to a difference in thickness between the cover layers are given according to formula (2):
                              W          40                =                              Δ            ⁢                                                  ⁢                          t              ⁡                              (                                                      n                    2                                    -                  1                                )                                      ⁢                          NA              4                                            8            ⁢                          n              3                                                          (        2        )            where Δt is the difference in thickness between the cover layers, and n is the refractive index of the cover layers (see, for example, S. Kubota, “Aplanatic condition required to reproduce jitter-free signals in optical disk system”, Appl. Opt. Vol. 26, pp. 3961–3973 (1987) (hereinafter referred to as the Kubota paper)).
For example, when the permissible spherical aberration (W40) is λ/4, the permissible fluctuation value (Δt) of the cover layer thickness in the above DVD playback device is ±27 μm. As for the double-layered disk used in the DVD playback device, the distance between data-recording layers is restricted to about 40 μm so as to fall in the above tolerance.
Recently, there has been disclosed a technique in which a high-capacity optical disc recording/playback device is achieved by shortening the wavelength of the light source and increasing the numerical aperture of the objective lens.
In this technique, a blue-violet semiconductor laser and an objective lens with a numerical aperture of 0.85 are used, and a recording capacity over 23 gigabytes is achieved in a DVD-size optical disk. On the other hand, according to formula (2), the accuracy of the cover layer thickness needs to be within ±4 μm.
However, if the same double-layered disk as in the DVD device is used in an optical disk recording/playback device using a high numerical aperture lens, in order to prevent interlayer interference of data signals, it is necessary to secure an interlayer distance of about 20 μm, which does not fall within the tolerance (±4 μm) for the cover layer thickness.
In order to deal with different cover layer thicknesses, there is disclosed a technique in which the spherical aberration is corrected by an expander lens (see, for example, Japanese Unexamined Patent Application Publication No. 2000-131603).
In addition, a more effective technique using a liquid crystal element is disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 10-020263 and Japanese Unexamined Patent Application Publication No. 2001-331963). The liquid crystal element has, for example, a concentric electrode pattern, and, according to the voltage applied to electrodes, it can generate a wavefront substantially equivalent to the degree of correction of the spherical aberration caused by the thickness error of the cover layers (see, for example, M. Iwasaki, M. Ogasawara, and S. Ohtaki, “A new liquid crystal panel for spherical aberration compensation”, Tech. Digest of Optical Data Storage Topical Meeting, SPIE 4342, pp. 103–105 (2001) (hereinafter referred to as the Iwasaki paper)).
When an objective lens with high numerical aperture is applied to recording and playback of a multi-layer disk medium, it is necessary to use a technique in which the focal point of the focal spot is selectively moved and controlled with respect to the target data-recording layer in addition to the above-described correcting technique.
There is disclosed a technique in which the above correcting unit is optimized in advance for the target data-recording layer when the pull-in operation of focus control and the focus movement are performed (see, for example, Japanese Unexamined Patent Application Publication No. 2002-100061). In addition to the optimizing technique of the above correcting unit, there is disclosed the application timing of acceleration pulses necessary for the focus movement (see, for example, Japanese Unexamined Patent Application Publication No. 2002-157750).
In an actual multi-layer optical recording medium, there is a possibility that a manufacturing error concerning the cover layer thickness occurs. Additionally, in the above-described correcting unit itself, there is a deviation of the degree of correction made to the applied voltage. Therefore, after completing the focus control pull-in operation, there needs to be a fine-tuning so that the degree of spherical aberration correction is optimal.
In particular, there are disclosed: a technique in which a playback signal from a data-recording medium is tuned to the optimal signal by using the jitter value represented as a fluctuation of the data edges with respect to a playback clock (normally, a PLL Clock: Phase-Locked Loop Clock), the signal amplitude, or the error rate (see, for example, K. Osato, I. Ichimura, F. Maeda, K. Yamamoto, and Y. Kasami, “Progress in optical disk recording with over 20 GB of capacity”, Tech. Digest of Optical Data Storage Topical Meeting, Whistler, pp. 15–17 (2000) (hereinafter referred to as the Osato paper)); and a technique in which there is provided an automatic correcting mechanism based on a spherical aberration error signal generated from the return light intensity from a data-recording medium (see, for example, T. Shimano, M. Umeda, and T. Ariyoshi, “Spherical aberration detection in the optical pickups for high-density digital versatile discs”, Jpn. J. Appl. Phys. 40, pp. 2292–2295 (2001) (hereinafter referred to as the Shimano paper)).
On the other hand, the wavefront aberration W20 caused by a focus error (Δz) is proportional to the square of the numerical aperture of a lens, and described according to formula (3):
                              W          20                =                              1            2                    ⁢          Δ          ⁢                                          ⁢          z          ⁢                                          ⁢                      NA            2                                              (        3        )            
However, in a multi-layer optical disk device using the above-described high numerical aperture objective lens, since the focal depth of the focusing objective lens is generally shallow, a focus control error signal is discontinuous between data-recording layers. Therefore, there exists a problem in that it is difficult to determine the switching timing between acceleration and deceleration.
Therefore, in comparison with, for example, a known double-layered DVD device, interlayer movement of a light spot cannot be achieved easily.
It is an object of the present invention to provide an optical recording medium handling device and a focus controlling method for the same that can perform interlayer movement from one data-recording layer to another data-recording layer appropriately and easily with respect to a recording medium having a plurality of data-recording layers.